A typical compressor employs a reciprocal piston for compressing a gas in a cylinder. A similar apparatus having basically the same structure as the compressor may be used for expanding a gas in a cylinder (hereinafter referred to as gas expander). In such compressor and expander, it is most important on one hand to keep the piston closely in contact with the cylinder to suppress gas leak from the cylinder as much as possible. On the other hand piston must be loose enough to reduce the friction between the piston and the cylinder for smooth operation of the piston.
A known approach to attain these seemingly opposite objectives is to provide the piston and the cylinder with a lubricant forming a thin layer between them. This lubricant layer only decreases the friction, but also helps seal the gap between the piston and the cylinder. However, any lubricant can evaporate into the gas under compression/expansion of the gas in the cylinder. The diffused lubricant will degrade thermal properties of the refrigerant and some components of the apparatus in the long run.
In order to circumvent such deterioration caused by the lubricant, it is necessary to keep the gas clean. This may be attained by the use of a lubricant-free compressor/expander, in which the piston is reciprocated without lubricants. Known lubricant-free apparatuses, however, have disadvantages that the piston-cylinder system are likely to have large friction and thus will suffer from quick wear, thereby shortening the life of the apparatus. In order to minimize such friction in the lubricant-free piston-cylinder system, the system must be designed to allow for a uniform gap of about 10 microns and yet to strictly suppress transverse fluctuation of the piston rod during its reciprocal motions, which is a burden in design and manufacture of the apparatus.